In 3GPP LTE networks, a base station (i.e. evolved NodeB, eNB) of a Radio Access Network (RAN) transmits data and signalling between a core network (CN) and User Equipment (UEs) located within the base station's coverage area.
Developments in communication networks have seen increased deployment of so called ‘small’ cells operated by Low Power Nodes (LPNs), such as pico eNBs, femto cells, Home eNBs (HeNBs) or the like, which cells have a smaller coverage area than existing macro cells operated by a higher power macro base station. Networks comprising a number of different cell types, for example a network comprising a macro cell and a femto cell, are referred to as Heterogeneous Networks, or HetNets.
More recently the need to make further enhancements to small cells using low-power nodes, has been identified as one of the most important topics for further development of 3GPP standards compliant communication systems in order to enable such communication systems to cope with increases in mobile traffic especially for hotspot deployments in indoor and outdoor scenarios. According to this interest in small cell enhancements, scenarios and requirements for small cell enhancements were studied and captured in a 3GPP technical report (3GPP TR 36.932) the contents of which are herein incorporated by reference. TR 36.932 defines a low-power node as generally meaning a node whose transmit power is lower than that of macro node and base station classes. For example, as indicated above, both pico eNB and femto HeNBs are considered to be low power nodes.
Currently, the average geographic density of macro cells is 5-7 macro cells per square kilometre. However, it is predicted that the number of small cells in urban areas will reach 40 small cells per square kilometre. The increasing geographic density of small cells presents challenges in achieving successful and efficient handover procedures in HetNets. In a densely deployed small cell scenario for example, the number of small cells a piece of user equipment (UE) such as a mobile (‘cellular’) telephone or other similar mobile device should detect and measure for the purposes of mobility management (e.g. management of the smooth transfer from one cell to another) can be much larger than that of other macro cell or less dense HetNet scenarios.
Accordingly whilst, in the conventional LTE systems up to the latest Release 11, the use of Synchronisation Channels (SCH) which consist of a Primary Synchronisation Channel (P-SCH) and a Secondary Synchronisation Channel (S-SCH) can be used to help the UE to search, acquire, and synchronise with an eNB, it is generally accepted that the SCHs will be unable to cope with such a densely populated cell deployment scenario. Specifically, there are a number of other issues that need to be considered for a densely populated small cell environment including, for example: how to ensure efficient use of power; the speed of the search for/discovery of a small cell; the ability to identify the cells; and/or the ability to acquire measurement results for the purposes of handover.